Optimization of beam optics and strategies for focusing the multi-beamlet accelerator of the MITICA injector

The MITICA experiment will be the full scale prototype of the neutral beam injectors to be installed on the ITER device. Its aim is to demonstrate and optimise a 16 MW neutral beam power, obtained by neutralization of a 56 A deuterium negative ion beam D- accelerated to an energy of 1 MeV. Since the current expected density available at the source is around 300 A/m2, a large extraction area is needed in order to obtain the required current, so that the beam is formed by the superposition of 1280 beamlets extracted from the source by means of a set of grids biased at different potentials. In order to satisfy the ITER requirements on beam aiming and focusing over long distances, the optimization of both single and multi beamlet optics is required. This calls not only for a careful perveance matching (balance of extracted current and grid voltages on each beamlet), but also for the compensation of the electrostatic interaction among beamlets, and with support structures. The paper describes the work carried out on this topic, paying particular attention to the multi-beamlet analyses. The most promising method to steer the beamlets are studied and a comparison among different design configurations is made.